The heat transfer surfaces in elastomer reactors, including butyl rubber reactors, foul due to the progressive accumulation of elastomer during operation. This accumulation leads to a decrease in the heat transfer coefficient between the reactor slurry and the refrigerant (e.g., ethylene) and consequently an increase in the polymerization/slurry temperature. Ultimately, the slurry temperature becomes too high for polymerization to continue because of reduced slurry stability and reduction of the molecular weight of the polymer. The reactor then must be taken off line, warmed to above ambient temperature and solvent washed before being refilled with feed and chilled back down to the polymerization temperature.
As disclosed in U.S. Pat. No. 5,417,930, hereby incorporated by reference in its entirety, in elastomer synthesis techniques, liquid chemical raw materials are catalytically converted into elastomeric solids or semi-solids, as in the production of synthetic rubber from low boiling hydrocarbons. A reaction mixture is circulated as a slurry in a reactor into which reactants and catalysts are injected, and product withdrawn. For example, isobutylene can be polymerized with a diolefin in the presence of a Friedel-Crafts type catalyst (e.g., an aluminum halide catalyst) dissolved in a diluent of low freezing point (e.g. at temperatures of about −100° F. to −160° F.) to produce butyl rubber. For this reaction, a back-mixed reactor can be employed; such as a one-tube pass system as disclosed in U.S. Pat. No. 2,474,592, which is hereby incorporated by reference. Such a reactor generally has a vertically oriented elongate vessel formed by an enclosing side wall. Within the sidewall an axially mounted draft tube is provided of relatively large diameter, surrounded by a relatively large number of small diameter tubes which extend downwardly from an upper common plane to a lower common plane, where the upper and lower terminal ends of the small diameter tubes and draft tube, respectively, terminate. As is typical of many elastomer reactors and synthesis equipment, the outer walls of the reaction vessel form a jacket through which a liquid hydrocarbon refrigerant is circulated to remove the exothermic heat of reaction via heat exchange contact with the outer walls of the small diameter tubes, and wall of the central draft tube.
Polymer fouling can be a serious problem encountered in this type of reactor. Polymer deposits upon and fouls heat transfer surfaces within the reaction vessel; the polymer adhering tenaciously to the metal surfaces as a continuous film, and in severe cases as large masses of rubber. Although not entirely understood, this phenomenon is believed to be caused at least in part by local overheating. There is a tendency of the polymer to form or deposit on the reactor surfaces. This manner of polymer formation or deposition occurs when the polymer accumulates directly on the reactor surfaces, and is referred to herein as “film deposition” or “deposition.” The rate of polymer film deposition on the reactor surfaces is generally proportional to the rate of polymerization, whereas particle agglomeration depends more on the characteristics of the slurry, flow conditions, particle adhesion, etc. As the film deposition accumulates, the heat transfer coefficient between the reactor slurry and the refrigerant decreases, leading to an increase in the polymerization temperature of the reactor slurry. As the reactor slurry temperature increases, the polymerization process becomes less stable and it is more difficult to achieve the desired molecular weight of the polymer product.
Additionally, during carbocationic polymerization processes, there can be a tendency of the polymer particles in the reactor to agglomerate with each other and to collect on the reactor wall, heat transfer surfaces, impeller(s), and the agitator(s)/pump(s). This is referred to herein as “polymer agglomeration,” “particle agglomeration,” or “agglomeration.” The rate of agglomeration increases rapidly as reaction temperature rises. Agglomerated particles tend to adhere to and grow and plate-out on all surfaces they contact, such as reactor discharge lines, as well as any heat transfer equipment being used to remove the exothermic heat of polymerization, which is critical since low temperature reaction conditions must be maintained. Others have attempted to address these problems in reaction vessels. Several examples are US Patent Application Publication No. 2005/0095176 (Hottovy), US Patent Application Publication No. 2005/0277748 (Kimoto et al), and EP 0 107 127 A1 (Sumitomo), each of which are herby incorporated by reference in their entirety.
Hottovy discloses a loop reactor that can prevent the creation of fine particulates, or fines, during olefin polymerization wherein the process is suitable for the copolymerization of ethylene and a higher olefin. A first polymerization is generated that actually creates a film/coating on the reactor walls so that larger particulates formed during the desired polymerization are not broken or chipped by a non-smooth reactor wall.
Nonetheless, polymer fouling presents a serious problem and it has greatly limited the usefulness, as well as the efficiency of, elastomer reactors and synthesis equipment.
For example, it is reported in U.S. Pat. No. 2,999,084, hereby incorporated by reference, that “[c]ommercial experience has demonstrated that mass fouling is a limiting factor of prime importance with respect to the rate of production of tertiary isoolefin polymers in that fouling to an extent sufficient to inhibit adequate refrigeration will occur at erratic and unpredictable intervals within the range of about 10 to 90 hours.” “When this happens, it is necessary to ‘kill’ the reaction medium and clean out the reactor before resuming the polymerization reaction,” this normally requiring 10 to 20 hours. Polymer fouling results in poor heat transfer, and loss of efficiency in the process operation. If fouling is not controlled, the usefulness of the reactor is greatly curtailed.
Thus there is a need to reduce the fouling rate in elastomer reactors and synthesis equipment (e.g., butyl rubber reactors) in order to extend the time between reactor cleaning and washes and/or to increase the throughput of the reactor.